It has been disclosed that sticky polymers such as ethylene/propylene/diene monomer (EPDM) and polydienes such as polybutadiene (BR), polyisoprene (IR), and styrene-butadiene rubber (SBR) can be produced in gas phase polymerization processes. Such processes are disclosed, for example, in U.S. Pat. Nos. 4,994,534; 5,304,588; 5,453,471; 5,652,304; 5,858,903; 5,877,109; EP 0 657 647; and WO 96/04322 and 04323.
In general, a sticky polymer is defined as a polymer being particulate at temperatures below the sticking or softening temperature but forming agglomerates at temperatures above its sticking or softening temperature. In order to maintain such forming polymers in a fluidized state in a gas phase polymerization, an inert particulate material such as carbon black, silica, talc, clay, activated carbon black, and mixtures thereof can be employed. These inert particulate materials also serve to coat the polymer rendering it a non-sticky, free-flowing granule or particle as it exits the polymerization reactor.
More recently, it has been disclosed in WO 98/34960 that a "modified carbon black" can be employed for these purposes. A "modified carbon back" is defined on pages 4-5 of the reference. For the most part, this definition defines a modified carbon black as a silicon-treated or silica-coated carbon black. However, part (c) of the definition discloses that the modified carbon black can be "a carbon black having an attached organic group(s)." According to the reference (pages 10-18), this organic group is attached via the reaction of a diazonium salt with a carbon black. Such a substituted organic group imparts water dispersibility (pages 17-18). Other organic groups, according to the reference (pages 18-19), can include attached aromatic sulfide and aminophenyl groups. It is further suggested in this reference (pages 20-21) that the use of these modified carbon blacks in a gas phase polymerization will produce a polymer product containing them which, in turn, will result in an article (e.g., hose or tire) having improved properties. Gas phase polymerizations employing these modified carbon blacks and the suggested improvements are not demonstrated.
Additionally, WO 98/34960 does not address the problems discovered and solved by the inventors of this invention. In the present invention, the inventors have found that monomers (especially dienes, as well as any other organic species) have a strong tendency to be adsorbed by carbon black that is employed in a polymerization as the inert particulate material (aka fluidization aid). When this occurs, the monomer can also be absorbed in the polymer product. First, this phenomenon makes it extremely difficult to purge or otherwise remove unreacted monomer from the polymer in post reactor processing. That is, it is necessary to purge unreacted monomer for much longer periods of time under more stringent conditions to remove it because of its potential exposure to the environment and/or to humans. Second, the presence of a monomer residue adsorbed on the surface of the carbon black in the polymer product and/or the stress imposed in extended purging may reduce the polymer's carbon black reinforcing ability in a compounded article.
Thus, for these reasons, there needs to be provided a carbon black that not only can coat the forming polymer in a polymerization such as in a gas phase reactor so that it is fluidizable, but the carbon black must also be one that does not readily adsorbed or retain monomer such that an onerous amount of time is required for purging or removal. The present invention provides a way to reduce the retention/adsorption of undesirable monomer in the polymer product.